Conductive metal films provide interconnections between contacts of devices in an integrated circuit and between the circuit and the outside world. In a typical integrated circuit there is at least one layer of metal and often two or more layers of metal. While a number of metals are suitable for use in integrated circuits, aluminum is the metal most widely used for contacting devices and for interconnections of devices to the outside world. As such, a first level of metal, such as aluminum, provides ohmic contacts to surface contact regions of semiconductor devices. The ohmic contacts are separated from each other by one or more insulating regions. A second level of metal, also aluminum, provides an interconnection between one or more first level metal contacts and the outside world.
However, there are drawbacks to contacting a semiconductor surface with aluminum. One problem is the solubility of silicon in aluminum. Silicon tends to diffuse into aluminum. Over time the aluminum-silicon interface degrades, aluminum penetrates below the aluminum-silicon interface which may lead to junction failure. Another problem is metal migration. Under the application of voltage and current, the metal on the silicon migrates or pushes its way into the silicon. Such metal migration is accelerated by high voltages and high currents. If the metal migration is enough, the metal passes through a surface contact region to a deeper region where the metal will short out a junction between the two regions in the body of the device.
Many of the problems posed by metal migration and diffusion are solved by the use of a barrier metal layer between the silicon and the deposited aluminum. A typical barrier layer metal is titanium tungsten (TiW). TiW acts as an adhesive layer and adheres to both silicon and aluminum. In addition, it has the property of preventing the migration or diffusion of aluminum into silicon. Such barrier metal layers are widely used in integrated circuits. Other barrier metals include, but are not limited to, TIN,& Ti.
Certain integrated circuits also take advantage of the metal migration properties of aluminum deposited on silicon. Such devices are typically programmable devices and are typically diodes with Zener diodes being the preferred choice. The Zener diode, under normal biasing conditions, is back biased and appears to be an open circuit. During programming, the Zener is subjected to high currents and high voltages, resulting in Joule heating, which cause one of its aluminum contacts to a Zener surface region to migrate through the junction of the Zener diode and effectively short out the diode. Thus, with such metal migration programmable Zener diodes, a user has the opportunity to provide a programmable device. A Zener diode, in its initial state and back biased will appear as an open circuit. A Zener diode programmed and shorted out by metal migration will appear as a short circuit or at least a very low resistance path as compared to a back biased Zener diode.
However, when devices that rely upon metal migration programmable Zener diodes are provided with barrier metal layers, the Zener diodes lose their programmability since the barrier layer metal prevents the metal migration of aluminum into silicon and thereby prevents the programming of Zener type programmable devices.
Programmable Zener diodes are often used in application specific integrated circuits (ASIC). Such circuits may be individually designed for a particular application. Alternatively, ASICs are often constructed from a number of so-called cells. Each cell represents a design of one circuit that is compatible in operation and manufacture with other circuits or cells. Together the cells form a library of modular circuit designs available for creating an ASIC. In ASICs, it is often desirable to combine predetermined integrated circuit cells with individually programmable cells where such programmable cells are programmable by the user or programmable at the end of the manufacturing process. In the manufacture of such ASICs, barrier metals are now widely used in order to prevent aluminum from penetrating into the surface of the silicon. However, the presence of the barrier metal interferes with those ASICs that rely upon programmable Zener diodes.
It is an object of this invention to provide a programmable device in an integrated circuit with a barrier metal system.
It is also an object of this invention to provide a method and apparatus for programming a Zener diode in an integrated circuit having a barrier metal system.